The trade name Multilock is used to designate a string of sondes each comprising an anchor arm that is displaceable under drive from a spring between a retracted position that enables the string to be lowered down the borehole and an anchored position in which the arm bears against the wall of the borehole to hold the sonde in place. The arm is kept in the retracted position and the spring is kept compressed during lowering by locking means. At the desired depth, the locking means are released, and the arm is moved away from the body of the sonde towards its anchoring position by the spring relaxing. When the string is raised, the anchor arm rubs against the wall of the borehole, which wall may include roughnesses liable to catch the arm, thereby giving rise to damage or to the string being jammed downhole.
French patent application No. 2 672 692 discloses a string of small-sized sondes that includes hydraulically actuateable retractable anchor means connected to a common source of hydraulic pressure. The anchor means are constituted by arms moved away from the sonde body by a rack which is driven by the piston of a hydraulic actuator. The rack is fixed to the piston by means of a shear pin that is designed to break under the effect of an upwardly-directed force, thereby enabling the arm to be folded down when the sonde is to be pulled out from the borehole. The assembly formed by the rack and the piston must be removed from the body of the sonde in order to be repaired.